Many condensation or step-growth polymers, including polyesters, polyamides, polycarbonates, and polyurethanes are widely used to make plastic products such as films, bottles, and other molded products. The mechanical and physical properties of these polymers are highly dependent on their molecular weights.
In a life cycle, these materials may experience a synthesis process, followed by an extrusion step, and a final processing step which may be another compounding/extrusion operation followed by profile or sheet forming, thermoforming, blow molding, or fiber spinning, or they can be injection or otherwise molded in the molten state. Typically, all of these steps occur under high temperature conditions. In addition, in recent years, increased attention has been focused on improved methods of reclaiming and recycling the plastics made from these polymers, with an eye toward resource conservation and environmental protection. The processing steps involved in recycling these polymers also involve high temperatures.
In each one of these high temperature steps, particularly during the compounding/processing and reclaiming/recycling processes, some degree of polymer molecular weight degradation occurs. This molecular weight degradation may occur via high temperature hydrolysis, alcoholysis or other depolymerization mechanisms that are well known to occur in such polycondensates. It is known that molecular weight degradation negatively affects the mechanical, thermal, and rheological properties of materials, thus preventing them from being used in demanding applications or from being recycled in large proportions for their original applications. Today, recycled or reprocessed polycondensates with deteriorated molecular weights can only be used in very low proportions in demanding applications or in larger proportions in less demanding applications. For instance, due to molecular weight degradation, recycled bottle grade polyethylene terephthalate (PET) is mostly employed exclusively in fiber and other low end applications. Similarly, recycled polycarbonate from compact disk (CD) scrap, mostly goes to low end applications. For these reasons, the current recycling technologies are limited to a narrow range of applications.
Today, there exist a considerable number of processes in the art employed to minimize loss in molecular weight and to maintain or even increase the molecular weight of the polycondensates for processing or recycling. Most of these routes employ as main processing equipment either an extruder, a solid state polycondensation reactor, or both, in sequence, or similar equipment designed for melt or high viscosity material processing. As an instrumental part of any of these processes, chemical reactants known in the art as “chain extenders” are employed. Chain extenders are, for the most part, molecules containing more than two functional groups that are included as additives in the reactor or extruder during any or all of the described processing steps with the purpose of “re-coupling” polycondensate chains that have depolymerized to some degree. Normally, the chain extender has two or more functional groups that are reactive with the chemical groups formed during the molecular weight degradation process. By reacting the chain extender molecule with two or more polycondensate fragments it is possible to re-couple them (by bridging them), thus decreasing or even reverting the molecular weight degradation process. In the art there are numerous chain extender types and compositions, polycondensate formulations, and processing conditions described to this end. For example, U.S. Pat. No. 6,984,694, which is incorporated by reference, represents one type of chain extender.
However, chain extenders have met with limited success in solving the problem of molecular weight degradation in reprocessed polymers. The shortcomings of copolymer chain extenders can be attributed, at least in part, to the fact that they are produced by conventional polymerization techniques which produce copolymers of very high molecular weight, and when coupled with a polycondensate can dramatically increase the molecular weight leading to localized gelation and other defects with physical characteristics which limit their capacity to act as chain extenders.
Most of the known chain extenders require the use of a pre-dried polycondensate material, operation at high vacuum, and varying amounts of catalyst and stabilizers, to be employed during processing. Without these features the extent of molecular weight increase is limited and the resulting product shows lower molecular weight and less than desired properties.
As the functionality of the chain extender increases, so does the number of polycondensate chains that can be coupled onto each chain extender molecule, and, thus, its effectiveness in re-building molecular weight. However, it is easy to see that as the polyfunctionality of these chain extenders increase so does the potential for the onset of gelation. Those of skill in the art are familiar with the strong negative effects associated with extensive crosslinking on the degree of crystallinity and thus on the mechanical properties of a semi-crystalline polycondensate, as well as the negative implications of the presence of varying amounts of gel in any product. For example, the high degree of branching or complete cross-linking that leads to gelation is undesirable for injection-molded thermoplastics due to the extremely high melt viscosity of resulting compositions. As a result of these negative effects there is a limit for the amount as well as functionality that can be employed with these chain extenders. Therefore effective chain extension currently requires relatively large concentrations of lower functionality (<4 functional groups/chain) chain extenders. Also, the maximum amount of chain extender that can be safely employed is limited in order to produce non-gelled compositions.
The relatively high costs associated with these two limitations of the current art, render the re-processing or recycling of these polycondensates uneconomical. Still other disadvantages are associated with many presently available chain extenders. For example, one disadvantage of polycondensates is the presence of certain types of end functional groups. Some of these functional groups can cause degradation of polymer molecular weight, such as carboxylic groups in some types of polycondensates often catalyze degradation of polymers by hydrolysis.